1. Field of the Invention
The present invention is directed to a permanent magnet of the type SE-Fe-B that has the tetragonal phase SE.sub.2 Fe.sub.14 B as the principal phase, wherein SE is at least one rare earth element, including Y.
2. Description of the Prior Art A magnet of the above general type is disclosed, for example, in European Application 0 124 655 and in U.S. Pat. No. 5,230,751 that corresponds therewith. Magnets of the type SE-Fe-B exhibit the highest energy densities currently available. SE-Fe-B magnets manufactured by powder metallurgy contain approximately 90% of the hard-magnetic principal phase SE.sub.2 Fe.sub.14 B.
U.S. Pat. No. 5,447,578 also discloses SE-Fe-B magnets that contain SE-Fe-Co-B-Ga phases as admixtures.
One usually proceeds such in the manufacture of such SE-Fe-B magnets by mixing a SE-Fe-B base alloy with the a composition close to the SE.sub.2 Fe.sub.14 B phase and a binder alloy with a lower melting temperature. The goal is to set the structure of the SE-Fe-B sintered magnets of SE.sub.2 Fe.sub.14 B base alloys with inter-granular binders, while using optimally little binder alloy.
European Application 0 517 179 proposes the employment of binder alloys having the composition Pr20Dy.sub.10 Co.sub.40 B.sub.6 Ga.sub.4 Fe.sub.rest (in weight percent, this is Pr.apprxeq.35, Dy.apprxeq.20, Co.apprxeq.28, B.apprxeq.0.77, Ga.apprxeq.3.5).
The special characteristic of this Pr20Dy.sub.10 Co.sub.40 B.sub.6 Ga.sub.4 Fe.sub.bal binder alloy is that it is composed of four inter-metallic phases. SEM investigations have documented that all four existing principal phases contain B and Ga. These, namely, are phases of the types:
SE.sub.5 (Co, Ga).sub.3 PA1 SE(Co[sic], Fe, Ga).sub.2, PA1 SE(Co, Fe, Ga).sub.3 PA1 SE(Co, Fe, Ga).sub.4 Bx.
The melting temperatures of the phases lie at approximately 560.degree. C., 980.degree. C., 1060.degree. C. and, respectively, 1080.degree. C. The phase 1/3 and 1/4 boride in fact have relatively high melting temperatures, but it is important that these lie just below the sintering temperature or, respectively, that they become molten at the sintering temperature. The phases 1/2, 1/3 and the 1/4 boride are ferromagnetic or ferrimagnetic with Curie temperatures of 110.degree. C., 340.degree. C. and, respectively, 375.degree. C.
It has now turned out that the proportion of this binder alloy in the mixture of the base alloy must lie within 7-10 weight %. In this mixing range, sinter densities of approximately .rho.&gt;7.55 g/cm.sup.3 are achieved only at sintering temperatures above 1090.degree. C. These sinter densities roughly correspond to 99% of the theoretical density. Outside this mixing range, the sinterability and, thus, the remanence that can be achieved are considerably influenced. The grain growth is highly activated in the magnets with a proportion of this binder alloy of more than 10 weight %, but the pores are not closed. The consequence is the formation of a structure with anomalously large grains (&gt;50 .mu.m) and with high porosity as well as with low sinter densities. Given lower proportions of binder alloy, the amount of the fluid phase is accordingly not adequate for the densification.